Investigation of interaction between factor VIII (fVIII) and von Willebrand factor (vWF) has been undertaken along several lines. First, the ability of vWF to block the interaction of factor VIII with antibodies to factor VIII has been studied by using the Bethesda assay. Inhibition of factor VIII activity by the antibodies is greatest when factor VIII preparations containing little or no vWF are tested (approx. 12,000 BU), decreases to about 5,000 BU for ratios of fVIII:vWF seen in plasma, and to 3500 for preparations with 3 times more vWF than fVIII. Ongoing work is intended to determine if this vWF effect is due to a vWF effect on the assay or represents blocking of the antibody by vWF. Second, the effect of vWF on factor VIII assays is being studied. Data generated thus far indicate that the vWF:Ag level must be at least 50% or normal for full expression of FVIII:C. And, third, the association of FVIII:Ag with individual vWF multimers is being investigated by SDS-agarose electrophoresis using radiolabelled anti-FVIII antibodies. Preliminary data indicate that FVIII is found in association with the small and intermediate size multimers.